Wet strength is a desirable attribute for many disposable paper products that are required to maintain their wet integrity for an extended period of time prior to and during their intended use. Such products include toilet tissue, diapers, personal care products, and pre-moistened articles such as baby wipes and household cleaning wipes.
Permanent wet strength, however, is often an unnecessary and undesirable characteristic in paper products. Many paper products are disposed of after brief periods of use into landfills, incinerators, etc. which is inconvenient and can pose a significant burden on the solid waste stream. It is therefore preferable in many cases to direct used paper products to municipal sewage treatment facilities or private septic systems. Unfortunately, this procedure is often not available if the product is “unflushable.” Clogging of sewage and septic systems can result if the product permanently retains hydrolysis-resistant strength properties. To address this problem, efforts have been undertaken to make binders that will provide paper products with sufficient “temporary” wet integrity in the presence of saline solutions, but minimal integrity when exposed to large amounts of waste water, such that they traverse plumbing and disintegrate in sewage and/or septic systems.
Specifically, it has been attempted to produce disposable fibrous products which maintain a relatively high wet strength in the presence of solutions with elevated ion concentrations, but become more dispersible when in contact with solutions having a lower ion concentration. These ion sensitive, water dispersible polymer formulations are well known in the art. There is disclosed, for example, in U.S. Pat. No. 6,429,261 to Lang et al. a polymer formulation comprising a triggerable copolymer of acrylic acid, NaAMPS, butyl acrylate and 2-ethylhexyl acrylate as well as a non-crosslinkable co-binder emulsion polymer comprising poly(ethylene-vinyl acetate), wherein the polymer formulation is insoluble in a neutral salt solution containing at least about 1 weight % salt and wherein the triggerable copolymer is soluble in water containing up to about 200 ppm of one or more multivalent ions. The polymers in Lang et al. are typically prepared by solution polymerization.
U.S. Pat. No. 6,683,129 to Eknoian discloses salt-sensitive binders in aqueous emulsions comprising methacrylic acid and acrylate monomers such as butyl acrylate and 2-ethylhexyl acrylate. The ion sensitive binders are applied as emulsion compositions.
Both references discussed above produce anionic binders, Lang et al. '261 produce ion sensitive anionic binders by solution polymerization, whereas Ekonian produces anionic binders by emulsion polymerization. These approaches are typical in salt-sensitive binder applications. These binders are called anionic binders because the free carboxylic functional group in these binders are neutralized with bases such as sodium hydroxide, calcium carbonate or ammonia to form the corresponding carboxylate salts. However, anionic binder compositions, i.e. anionic salt triggerable polymers, exhibit certain undesirable properties. For instance, anionic salt triggerable polymers fail to disperse in water containing more than about 15 ppm Ca2+ and/or Mg2+. Thus, when nonwoven webs containing these polymers are placed in hard water, which typically contains more than about 15 ppm Ca2+ and/or Mg2+, they maintain a tensile strength greater than 30 g/in, which negatively affects the dispersibility of the web. The reason for this is attributed to the fact that the carboxy functions in these binders interact with either calcium and/or magnesium ion in the hard water thereby slowing the dispersibility.
The cationic binders on the other hand would result in instant dispersibility of the binders in any water including hard water. This is because of the fact that the cationic binders do not feature any functional groups that could interact for example with either calcium and/or magnesium ions present in hard water. Such uses of cationic binders in wet wipes are not yet known except for a few examples wherein the cationic binders are made by solution polymerization employing undesirable solvents as discussed further below.
However, cationic polymers having utility either as thickening agent or as a hair conditioning resin composition has been reported in the literature. For instance, U.S. Pat. No. 5,608,021 to Uchiyama et al. discloses a cationic polymer thickener. The cationic thickener is prepared by solution polymerizing a monomer composition comprising 15 to 85% by weight of at least one of acrylic monomer having an amino group and methacrylic monomer having an amino group, 20 to 80% by weight of vinyl monomer, 1 to 20% by weight of monomer having at least one of acryloyl group and methacryloyl group and 0.1 to 20% by weight of crosslinkable vinyl monomer.
European Patent Application No. 0 201 342 of Kubota et al. discloses hair conditioning resin compositions, which comprises a copolymer resin modified with a zwitterionizing agent. The resin is prepared by a solution polymerization of amino functionalized alkyl acrylates or methacrylates, amino functionalized alkyl acrylamides or methacrylamides, vinyl acetate-type monomers and other vinyl monomers. The polymerization is generally carried out in a hydrophilic solvent.
U.S. Pat. No. 7,070,854 to Chang et al. discloses triggerable, water dispersible cationic polymers. The cationic polymers as disclosed therein are made by solution polymerization comprising a copolymer containing quaternary ammonium groups and are employed as binder materials in the production of a nonwoven fabric.
U.S. Pat. No. 7,141,519 to Bunyard et al. discloses ion triggerable, water dispersible cationic polymers. The cationic polymers disclosed therein are made by solution polymerization and comprise either terpolymers or copolymers of quaternary ammonium group containing acrylates or methacrylates, alkyl acrylates or methacrylates and vinyl acetates, and the like. These cationic polymers have been used as binder compositions in water-dispersible personal care products, such as wet wipes.
United States Patent Application Publication No. US 2007/0254543 of Bunyard et al. discloses dispersible wet wipes comprising a nonwoven material. The nonwoven material comprises a fibrous material and a binder composition. The binder composition includes a cationic ion sensitive emulsion polymer, which is synthesized in-situ from a cationic polymer prepared from solution polymerization.
In contrast to the above procedures, emulsion polymers converted into water soluble polymers, whereby the emulsion polymer is put into solution by lowering the pH to acidic pH have not heretofore been used as salt-sensitive binders. Emulsion polymerized, acid-solubilized polymers have been employed primarily in applications such as thickeners. Thickeners are added to aqueous systems to increase the viscosity to a desired level and are frequently added to materials such as paints, polishing and cleaning compositions, pharmaceuticals, among others.
European Patent Application No. 0 055 801 of Fink et al. discloses a process for thickening aqueous systems by mixing the aqueous system with an aqueous dispersion of a synthetic polymer and adjusting to a pH value at which the synthetic polymer is at least colloidally soluble. The process involves mixing of the aqueous system with an aqueous dispersion of a synthetic polymer composed of: A) 5-100% by weight of an unsaturated radically polymerizable monomer with at least one basic nitrogen atom, B) 0-95% by weight of an unsaturated radically polymerizable comonomer which has at most limited solubility in water, C) 0-30% by weight of a water-soluble non-basic unsaturated radically polymerizable comonomer and has a molecular weight of at least 500,000 and is at least colloidally soluble at a pH of below 7.
United States Patent Application Publication No. US 2005/0075445 of Confalone et al. discloses a coating composition for fibrous substrates made up of 0.5 to 25 percent by weight of a water-insoluble emulsion polymer comprising from greater than 0.4 to 3 mole percent of one or more cationic monomer units and at least 50 mole percent of at least one vinyl ester monomer; 25 to 75 percent by weight pigment; cationic surfactant; and water.
Emulsion polymerization offers several advantages in the production of nonwoven binders. For example, emulsion polymerization is cost effective and environmentally safe, allowing for the production of a high-solids composition without the necessity of removing unwanted solvent. However, the present Applicants have observed that emulsion binders may not “trigger” as well as solution binders. “Triggering” is a critical mechanism in salt-sensitive applications whereby the polymer becomes insoluble in concentrated salt solutions, yet remains dispersible when in contact with solutions containing a lower concentration of ions. Also, the film-forming mechanism in emulsion polymers is different and less predictable than solution polymers. On the other hand, solution polymerization is disadvantaged from a processing standpoint because the solvent must be removed and the resulting compositions typically do not achieve as high of a solids content as can be produced with emulsion polymerization.
Thus, despite the contributions in salt-sensitive binders and products incorporating them, there still exists a need for high quality salt-sensitive binders which may be produced safely and efficiently.